(1) Field of the Invention
The present invention relates to a four wheel drive vehicle, particularly an agricultural tractor, comprising a transmission system for driving front wheels at a higher speed than rear wheels to be capable of making a turn with a small turning radius during an operational run.
(2) Description of the Prior Art
A four wheel drive vehicle is disclosed in U.S. Pat. No. 4,669,559, which comprises a front wheel drive line including a front wheel change speed mechanism for producing two, high and low, speeds. When the low speed is selected, a standard speed mode is produced wherein a front wheel speed is equal to a rear wheel speed. When the high speed is selected, a front wheel acceleration mode is produced wherein the front wheel speed is greater than the rear wheels speed.
The term "front wheel speed" used herein refers to a vehicle speed produced by driving the front wheels, and may generally be regarded as a peripheral speed of the front wheels. The same applies to the "rear wheel speed".
To enable this type of vehicle to make a turn with a small turning radius, switching is made for the front wheel acceleration mode in response to a steering angle of the front wheels exceeding a certain angle. A sudden acceleration of the front wheels for making a turn is dangerous when the vehicle is traveling at high speed along a road, for example. The vehicle, therefore, includes check means to prevent the switching for the front wheel acceleration mode when a propelling drive transmission is in a high speed position. This construction allows the front wheel change speed mechanism to function only when the vehicle is traveling at low speed, and does not provide a solution to the problem of how the vehicle may be able to make a turn safely and efficiently when traveling on the road at high speed.